PMF (Polarization Maintaining Optical Fiber) is a kind of special optical fiber that realizes the single-mode transmission characteristic of light while maintaining its linear polarization state. Since PMF has the effect of stress birefringence, therefore PMF can maintain the polarization state well when transmitting linearly polarized light. The linear polarization maintaining capability of PMF is of utility value in many polarization-dependent applications (such as multidimensional multiplexed coherent communications, optical fiber gyro technology, current mutual inductance technology, optical fiber hydrophones and polarization sensing).
When PMF is used in the optical fiber gyro, the PMF will be wound into optical fiber ring for the application. Optical fiber ring is the core component of the optical fiber gyro, the performance of the optical fiber ring directly determines the performance of optical fiber gyro, the length of optical fiber in optical fiber ring determines the accuracy of optical fiber gyro, and the performance of optical fiber itself determines the performance of environmental adaptability of the optical fiber gyro. The use of the environment and the process of optical fiber gyro adopting PMF are not the same with general communication optical fiber, the key of optical fiber gyro adopting PMF is focusing on optical fiber's performance indicators such as mechanical, optical polarization and environmental adaptability (mainly including room temperature crosstalk, full temperature crosstalk, normal temperature loss, full temperature additional loss change, mode field diameter, beat length, tension screening, cladding diameter, coating diameter, etc.).
The categories of PMF mainly include geometrical birefringence polarization maintaining optical fiber and stress birefringence polarization maintaining optical fiber. The geometrical birefringence polarization maintaining optical fiber is mainly elliptical core polarization maintaining optical fiber, when the geometrical birefringence polarization maintaining optical fiber is used, it uses the geometrical asymmetry of the core to realize the birefringent effect for controlling the linear polarization state of the light. The stress birefringence polarization maintaining optical fiber mainly includes tie-type polarization maintaining optical fiber, elliptical cladding type polarization maintaining optical fiber and Panda-type polarization maintaining optical fiber; when the birefringence polarization maintaining optical fiber is used, the material whose coefficient of expansion has a large difference from the cladding material is introduced into the cladding to form the stress zone, the birefringence effect is achieved by compressing the mode field of the light transmitted by the core by stress to maintain the linear polarization properties of the light.
Since the Panda-type polarization maintaining optical fiber has been successfully introduced, driven by the technical requirements of the optical fiber gyro, the evolution of cladding diameter/coating diameter of the Panda-type polarization maintaining optical fiber is 200 μm/400 μm, 125 μm/245 μm and 80 μm/165 μm. When the cladding diameter of the Panda-type polarization maintaining optical fiber is 80 μm, its coating diameter can be as small as about 165 μm, compared with the conventional optical fiber of 125 μm/245 μm, the coating diameter (165 μm) of the Panda-type polarization maintaining optical fiber of 80 μm/165 μm is decreased by more than 30%, the volume is decreased by more than 50%.
In summary, the optical fiber gyro has a smaller overall dimension when the Panda-type polarization maintaining optical fiber of 80 μm/165 μm acting as its optical fiber ring, and then brings a series of benefits for the application of the optical fiber gyro, among which the effect of improving the temperature performance of optical fiber ring is the most remarkable. However, with the further development of optical fiber gyro technology, and the development and application of current mutual inductance technology for smart grid, the optical fiber ring needs better temperature performance and smaller size.
Faced with this new requirement, it is necessary to develop a polarization maintaining optical fiber of smaller overall dimension; conventionally, the method of reducing the overall dimension of the polarization maintaining optical fiber is generally simply to reduce the diameter of the fiber directly. However, because of the smaller diameter and thinner coating of the optical fiber, further lead to a puzzle that the optical fiber has lower resistance to external interference, so the traditional method of reducing the overall dimension of polarization maintaining optical fiber is difficult to be applied to the optical fiber gyro with high precision demand.
Therefore, the urgent problem of many application techniques such as optical fiber gyro, mutual inductor of high-performance is how to develop a polarization maintaining optical fiber which not only has good fusion performance with conventional communication optical fiber and polarization maintaining optical fiber, but also has a much smaller volume, and also has excellent geometry and optical properties.
It is well known that when there is time-varying temperature perturbation of a section of optical fiber in an optical fiber ring, two back-propagating light waves pass through the optical fiber at different times and undergo different phase shifts due to temperature disturbances, this phase shift is a nonreciprocal phase shift (i.e. the Shupe effect) caused by ambient temperature. The measurement error of optical fiber gyro caused by the Shupe effect is known as the Shupe error.
The temperature change rate of certain section of optical fiber is usually caused by the temperature gradient of the inner and outer sides of the optical fiber ring caused by the ambient temperature, the temperature rate sensitivity which characterizes the Shupe error is also called temperature gradient sensitivity. The offset drift of optical fiber gyro caused by ambient temperature is caused by the rate of temperature change, since the Shupe error is not distinguishable from the Sagnac effect phase shift caused by rotation, it is necessary to take measures to suppress the temperature drift of the optical fiber gyro.
Now the method of suppressing the temperature drift of optical fiber gyro is as follows: the optical fiber ring is wound by symmetrical winding method such as quadrupole or octupole, and so on, the order of the adjacent two pairs of optical fiber layers is opposite to compensate the radial temperature field gradient. This method is very effective to suppress the Shupe error in optical fiber gyro, but due to the limitations of the process equipment, the state of optical fiber ring winding is still non-ideal, in medium and high precision optical fiber gyro applications, the residual temperature drift is still not negligible.
The small-diameter polarization maintaining optical fiber can suppress the temperature drift of the optical fiber gyro, and the cladding diameter of the small-diameter polarization maintaining optical fiber is 80 μm or less, and the coating diameter ranges from 90˜140 μm. Compared with polarization maintaining optical fiber with 80/165 μm (cladding diameter/coating diameter), the bend performance of the small-diameter polarization maintaining optical fiber is better; The same length of the optical fiber can be wound a ring of smaller size, while the number of layers in the gyro structure is also reduced, the temperature difference between the inside and outside the optical fiber ring decreases, which can effectively suppress the Shupe error of optical fiber gyro, and improve the full temperature performance of the optical fiber gyro. At the same time, as for the optical fiber gyro with determined geometric dimensions, by using small-diameter polarization maintaining optical fiber, a longer optical fiber ring can be winded, the limit accuracy of optical fiber gyro can be greatly improved under the same dimension, and the miniaturization application requirements of medium and high precision optical fiber gyro can be met.
However, with the continuous reduction of geometry dimension of optical fiber, how to reduce the cladding diameter and coating diameter of optical fiber while maintaining the excellent performance of polarization maintaining optical fiber has become a major problem in optical fiber design and manufacturing process.